Previously, position measurement devices that measure object positions using ultrasound are known. The following Non-Patent Document 1 describes a position measurement device that measures a distance to an object using ultrasound and calculates three-dimensional coordinates of an object based on the distance measured. In the position measurement device described in the following Non-Patent Document 1, an ultrasonic wave is emitted from a ground to an upward direction, and a reflected wave, which is reflected from a model airplane flying above, is received with three receivers. Next, this position measurement device determines reception timing of the reflected wave at each receiver based on a received wave of each receiver. Subsequently, based on the timing thus determined, a calculation is performed to obtain a propagation distance of the emitted ultrasonic wave that is reflected from the model airplane and returned to each receiver. Further, the position measurement device calculates three-dimensional coordinates of the model airplane based on the three propagation distances obtained at the respective receivers.    Non-Patent Document 1: IMAI Kensuke, and four others, “Construction of Three-dimension Measurement Systems of the High-speed Moving Object Using Ultrasonic Sensor”, Research Report, Kanto Gakuin University Kogakukai, March 2007, Vol. 50, Second Issue, pages 67-73.
The foregoing received wave includes various noises. Further, the received wave may sometimes include an ultrasonic wave reflected from an object other than a target object of measurement as noise. Thus, the reception timing of the reflected wave may be determined incorrectly if it is determined that the reflected wave is received when level of the received wave exceeds a threshold. This leads to an error in determining an arrival time, a time period starting from the emission of ultrasonic wave to the arrival at the receiver after being reflected from an object, thereby adding an error to the propagation distance calculated using this arrival time. Further, the three-dimensional coordinates are calculated using the respective propagation distances of the three receivers. Thus, errors of the three propagation distances are superimposed in calculation results. Accordingly, the three-dimensional coordinates that are obtained as the calculation results become less accurate.